I googled the two word together but could not find anything. I have never used it. Is there any necessity for it although we have main.storyboard.
They are two completely different things. The launch screen is what first appears when the user taps the app icon before the app is finished launching. It shows a single, static screen. It can't be dynamic and it can't use any custom classes or code. It's the replacement for launch images.
The main storyboard is what your app actually displays when the app is running. It contains your app, code, and logic.
When you run your program in Xcode simulator gets open and your application gets open then you can see black white screen for fews seconds before actuals developed screen
Blank screen - LaunchScreen.storyboard - Non editable
Developed Screen - Main.storyboard - Editable for which you can able to make changes
The launchscreen.storyboard (basically a loading screen) is what comes up when the user loads the application. How long it appears to the user depends on how long it takes for the user to load your application. You can change the duration of the launchscreen.storyboard but it is not recommended, generally users want the app to load as fast as possible
The main.storyboard is appears onto the screen AFTER the launchscreen.storyboard
When the app is being running in the Simulator/iPhone the 1st screen which is loaded in the memory after the user click the app icon is LAUNCHSCREEN.STORYBOARD and after the app is fully loaded, this is the time where MAIN.STORYBOARD comes into play, this is the screen which is editable you can add actions, outlets , other screen inside a View Controller, so the main work is being done inside MAIN.STORYBOARD and LAUNCHSCREEN.STORYBOARD is being used to display something static while the app gets fully loaded.
来源：https://stackoverflow.com/questions/34891481/what-is-the-difference-between-launchscreen-storyboard-and-main-storyboard